1. Field of the Invention
The present invention relates to a mineral wool plant substrate, more in particular to a mineral wool plant substrate comprising a foreign material in order to improve the properties of the mineral wool plant substrate.
2. Description of the Prior Art
Mineral wool plant substrates for plant growth are well-known in the art and consist of a coherent matrix of mineral wool. This coherent matrix is formed by collecting a layer of mineral wool fibres provided with a curable binder, so that after curing the mineral wool fibres are substantially not displacable relative to one another. If required for fast uptake of water this coherent matrix of mineral wool may be provided with a wetting agent.
As mineral wool may be used glass wool, stone wool, rock wool, or slag wool, and/or mixtures thereof.
The fibres may have an average diameter varying in between 1-10 .mu.m. For rock wool, the fibre diameter is on average about 4 .mu.m.
The density of the coherent matrix of mineral wool may be between 10-200 kg/m.sup.3, in general in between 30-80 kg/m.sup.3.
Such a coherent matrix of mineral wool has a form retaining property which is inherent due to the inorganic starting materials used therein. Furthermore, the water retaining capacity of these mineral wool plant substrates is very well controllable and predictable.
A disadvantage may be that during the plant growth, initially the matrix reacts basic so that the pH increases.
It is a desire of growers to alleviate the aforementioned disadvantage, but above that to have the possibility of controlling the exchange of nutrient cations.
Such properties are inherent to organic substances such as sphagnum and peat. However, these materials are unsuitable for use in mineral wool plant substrates because during culture they lose structure, and due to biological degradation of the organic substance, the water retaining capacity changes such that less air is included within the material resulting in acidification of the plant substrate.